1. Field of the Invention
The present invention relates to a biased condenser microphone, and more particularly, to an SMD (surface mounting device) type biased condenser microphone having two connecting terminals for a surface mounting.
2. Description of the Related Art
Generally, a condenser microphone includes a set of diaphragm and backplate provided with a capacitor (C), the capacitance of which is changed depending on a voltage bias factor and a sound pressure, and a junction field effect transistor (JFET) for buffering an output signal.
As an example of such a condenser microphone, there is a biased condenser microphone in which a bias voltage is supplied from the outside to form an electrostatic field between the diaphragm and the backplate.
FIG. 1A is an equivalent circuit diagram of a conventional biased condenser microphone. A microphone capsule 10 including a buffer IC 14 and a variable condenser 12 in a microphone unit is connected through three terminals 16-1, 16-2 and 16-3 to an external circuit. The first terminal 16-1 is used to connect an output portion of the buffer IC 14 through a resistor R1 to a power source Vdd and, at the same time, through a capacitor to a signal output portion, and the second terminal 16-2 is used to connect the buffer IC 14 to a grounding portion GND. Also, the third terminal 16-3 is used to supply a bias voltage to the microphone unit.
FIG. 1B is another equivalent circuit diagram of the conventional biased condenser microphone. The microphone capsule 10 including the buffer IC 14 and the variable condenser 12 in the microphone unit is also connected through three terminals 16-1, 16-2 and 16-3 to an external circuit. The first terminal 16-1 is used to supply a bias voltage through a resistor R2 to the microphone unit and the second terminal is used to connect an output portion of the buffer IC 14 through a resistor R1 to a power source Vdd and, at the same time, through a capacitor to a signal output portion. Also, the third terminal 16-3 connects the buffer IC 14 to the grounding portion GND.
However, since the conventional biased condenser microphone is provided with at least three terminals such as the bias terminal, the power and output terminal and the grounding terminal so as to interface with the outside, there is a problem that a direction of the circular condenser microphone should be checked upon a surface mounting process. Further, since a separate voltage device for supplying the bias voltage has to be provided to the outside of the microphone, it is difficult to miniaturize the microphone. Furthermore, since it has poor compatibility with an electret condenser microphone (ECM), which is generally used for connection with an external circuit, there is another problem that includes providing a printed circuit board (PCB), which has to be separately designed.